1. Field of the Invention
The present invention relates to a sheet feeder for feeding sheets, and an image forming apparatus such as a copier, a facsimile machine or a printer equipped with such a sheet feeder.
2. Description of the Related Art
Conventionally, a sheet feeder for feeding sheets has been used in an image forming apparatus. Generally, a sheet feeder is equipped in an image forming apparatus and feeds sheets stacked therein to the image forming apparatus while separating the sheets one by one from the uppermost sheet using a pickup roller. For examples, large-capacity sheet feeders externally attached to image forming apparatuses, sheet cassettes provided in main bodies of image forming apparatuses and the like exist as modes of sheet feeders.
In these sheet feeders, there are cases where an elevating plate movable upward and downward is disposed in a sheet accommodating portion. In a sheet feeder accommodating a relatively large amount of sheets, the elevating plate is frequently used. Sheets are stacked on this elevating plate. The elevating plate is elevated little by little so as to bring the uppermost one of the stacked sheets into contact with a pickup portion such as a pickup roller during a sheet feeding operation.
On the other hand, the elevating plate is lowered upon replenishing sheets. Some elevating plates are lowered straight to a lower limit position, and some elevating plates are gradually lowered every time sheets are replenished so that the elevating plate or the uppermost position of the stacked sheets is located at such a height as to enable easy sheet replenishment for users. After the completion of the sheet replenishment, the elevating plate is elevated to bring the uppermost sheet into contact with the pickup portion.
Japanese Patent No. 2931086 (D1) proposes an example of such a sheet feeder. D1 discloses that, in a large-capacity sheet feeder for feeding sheets stacked on a bottom plate of a tray movable upward and downward from the uppermost one by means of a sheet feeding mechanism, an inclined portion inclined upward in a sheet feeding direction is provided at a leading end portion of the tray bottom plate. By such a construction, compatibility between the large-capacity sheet feeder and a sheet cassette is improved, and an attempt is made to avoid a sheet jam and the like by enabling a sheet to be brought at a suitable angle to a feed roller present downstream of a pickup roller in the sheet feeding direction.
It has been a conventional and general practice to stack sheets in a sheet feeder such that the leading ends of the sheets in a sheet feeding direction are inclined upward or at least held horizontal. This is because, as disclosed in D1, the sheets are thought to be more stably fed with the leading ends of the sheets extending upward in view of the multiple feed of sheets and the like.
However, if the leading ends of the sheets are lifted up, sheets in an uppermost part of the sheet stack might be displaced backward. Then, it is no longer possible to reliably bring the uppermost sheet into contact with the pickup portion, thereby presenting a problem of being unable to precisely feed the sheets. This problem causes a sheet jam and the like.
Further, in order to precisely move the elevating plate provided in the sheet feeder capable of accommodating a large amount of sheets upward and downward, it is necessary to accurately detect the position of the uppermost part of the sheet stack. However, if the sheets in the uppermost part of the sheet stack are displaced backward, there is an addition problem of being unable to make an accurate detection in some cases.
Specifically, in the case of elevating the elevating plate to bring the uppermost one of the stacked sheets into contact with the pickup portion, the position of the uppermost sheet might be erroneously detected if the sheets are displaced backward. Such an error detection causes problems that a sheet other than the uppermost one is brought into contact with the pickup portion with the uppermost sheet left displaced backward and the elevating plate is excessively elevated. These problems cause a pickup error, a delay in feed timing, a sheet jam and the like.
On the other hand, upon gradually lowering the elevating plate such that the uppermost one of the already stacked sheets is at such a height enabling easy replenishment for a user during a sheet replenishing operation, the elevating plate might be stopped without being sufficiently lowered before reaching an originally determined amount of downward movement. This troubles the sheet replenishing operation of the user.
Since the upward inclined portion in the sheet feeding direction is provided at the leading end portion of the tray bottom plate in the sheet feeder of D1, sheets in the uppermost part of the sheet stack might be displaced backward. Further, the invention according to D1 aims to improve the compatibility between the large-capacity sheet feeder and the sheet cassette and to enable the sheet to be brought at a suitable angle to the feed roller, and does not consider the above problems at all.